Automatic identification (“Auto-ID”) technology is used to help machines identify objects and capture data automatically. One of the earliest Auto-ID technologies was the bar code, which uses an alternating series of thin and wide bands that can be digitally interpreted by an optical scanner. This technology gained widespread adoption and near-universal acceptance with the designation of the universal product code (“UPC”)—a standard governed by an industry-wide consortium called the Uniform Code Council. Formally adopted in 1973, the UPC is one of the most ubiquitous symbols present on virtually all manufactured goods today and has allowed for enormous efficiency in the tracking of goods through the manufacturing, supply, and distribution of various goods.
However, the bar code still requires manual interrogation by a human operator to scan each tagged object individually with a scanner. This is a line-of-sight process that has inherent limitations in speed and reliability. In addition, the UPC bar codes only allow for manufacturer and product type information to be encoded into the barcode, not the unique item's serial number. The bar code on one milk carton is the same as every other, making it impossible to count objects or individually check expiration dates.
Currently cartons are marked with barcode labels. These printed labels have over 40 “standard” layouts, can be mis-printed, smeared, mis-positioned and mis-labeled. In transit, these outer labels are often damaged or lost. Upon receipt, the pallets typically have to be broken-down and each case scanned into an enterprise system. Error rates at each point in the supply chain have been 4-18% thus creating a billion dollar inventory visibility problem. Only with radio frequency identification (“RFID”) does the physical layer of actual goods automatically tie into software applications, to provide accurate tracking.
The emerging RFID technology employs a radio frequency (“RF”) wireless link and ultra-small embedded computer chips, to overcome these barcode limitations. RFID technology allows physical objects to be identified and tracked via these wireless “tags”. It functions like a bar code that communicates to the reader automatically without needing manual line-of-sight scanning or singulation of the objects. RFID promises to radically transform the retail, pharmaceutical, military, and transportation industries.
The advantages of RFIDs over bar code are summarized in Table 1:
TABLE 1BarcodeRFIDNeed line-of-sight to readIdentification without visualcontactRead onlyAble to read/writeOnly a barcode numberAble to store information in tagBarcode number is fixedInformation can be renewedanytimeCategory level tagging only-no uniqueUnique item identificationitem identifierUnable to read if barcode is damagedCan withstand harshenvironmentUse onceReusableLow costHigher costLess FlexibilityHigher Flexibility/Value
As shown in FIG. 1, an RFID system 100 includes a tag 102, a reader 104, and an optional server 106. The tag 102 includes an IC chip and an antenna. The IC chip includes a digital decoder needed to execute the computer commands that the tag 102 receives from the tag reader 104. The IC chip also includes a power supply circuit to extract and regulate power from the RF reader; a detector to decode signals from the reader; a backscatter modulator, a transmitter to send data back to the reader; anti-collision protocol circuits; and at least enough memory to store its EPC code.
Communication begins with a reader 104 sending out signals to find the tag 102. When the radio wave hits the tag 102 and the tag 102 recognizes and responds to the reader's signal, the reader 104 decodes the data programmed into the tag 102. The information is then passed to a server 106 for processing, storage, and/or propagation to another computing device. By tagging a variety of items, information about the nature and location of goods can be known instantly and automatically.
Many RFID systems use reflected or “backscattered” radio frequency (RF) waves to transmit information from the tag 102 to the reader 104. Since passive (Class-1 and Class-2) tags get all of their power from the reader signal, the tags are only powered when in the beam of the reader 104.
The Auto ID Center EPC-Compliant tag classes are set forth below:
Class-1                Identity tags (RF user programmable, maximum range 3 m)        Lowest cost        
Class-2                Memory tags (8 bits to 128 Mbits programmable at maximum 3 m range)        Security & privacy protection        Low cost        
Class-3                Battery tags (256 bits to 64 Kb)        Self-Powered Backscatter (internal clock, sensor interface support)        100 meter range        Moderate cost        
Class-4                Active tags        Active transmission (permits tag-speaks-first operating modes)        30,000 meter range        Higher cost        
In RFID systems where passive receivers (i.e., Class-1 and Class-2 tags) are able to capture enough energy from the transmitted RF to power the device, no batteries are necessary. In systems where distance prevents powering a device in this manner, an alternative power source must be used. For these “alternate” systems (also known as active or semi-passive), batteries are the most common form of power. This greatly increases read range, and the reliability of tag reads, because the tag doesn't need power from the reader. Class-3 tags only need a 10 mV signal from the reader in comparison to the 500 mV that a Class-1 tag needs to operate. This 2,500:1 reduction in power requirement permits Class-3 tags to operate out to a distance of 100 meters or more compared with a Class-1 range of only about 3 meters.
Early field trials have shown that the currently available passive short-range Class-1 and Class-2 tags are often inadequate for tagging pallets and many types of cases. The problems with these passive tags are particularly severe when working with “RF-unfriendly” materials like metal (like soup cans), metal foils (like potato chips), or conductive liquids (like soft drinks, shampoo). It is very difficult if not impossible to consistently read case tags located in the interior of a stack of cases, as occurs in a warehouse or pallet. The existing passive tags are also inadequate to tag large or rapidly moving objects like trucks, cars, shipping containers, etc. Class-3 tags solve this problem by incorporating batteries and signal preamplifiers to increase range.
Conventionally, RF AM-Detector design uses either Schottky diodes or MOS devices configured as a full-wave bridging rectifier structure to extract the base-band signal out of an incoming amplitude-modulated (AM) RF carrier. However, the wireless link distance is proportional to the receiving sensitivity of an AM-Detector. Thus, AM-Detector circuits implementing traditional diodes or transistors are not sensitive enough for long range use. To increase the receiving sensitivity of the RF AM-Detector, either low barrier Schottky diodes with zero-bias forward voltage or low-threshold MOS devices with zero-threshold voltage are preferred candidates to implement the AM demodulation function operating at very low input RF signal levels such as the signal power range from −20 dBm to −50 dBm or even lower. These AM-Detectors will work quite well under the low RF input power in that very little signal strength is required to turn on the zero-bias Schottky diodes or zero-threshold MOS devices.
FIG. 2A illustrates a prior art full wave bridging rectifier AM-Detector 200 using Schottky diodes D. The circuit 200 is coupled to opposite ends ANTIN+, ANTIN− of an antenna. A differential signal comes into the circuit 200 as a positive (+) and a negative (−) signal. When node N1 goes high, node N2 goes low. Conversely, when node N1 goes low, node N2 goes high. The signal from ANTIN+, for example, passes through an AC coupled capacitor C and to node N1. When the capacitor C goes high, node N1 will also go high. When node N1 goes high, the voltage exceeds the threshold voltage of diode D1 and passes through to node N3 and to the output of the rectifier 200. The signal also passes through the filtering elements CL and RL and to ground (Gnd). As shown, node N4 is also coupled to Gnd, so the signal that passed through node N3 passes through Gnd, and then through node N4. From node N4, it passes through diode D3 and out to the negative antenna input ANTIN− at node N2. Conceptually, the signal thus makes a loop through the circuit and antenna. The same thing happens in the reverse direction when the signal at node N1 goes low and the signal at node N2 goes high.
The output of the AM-Detector 200 is a rectified signal that is ready for processing by circuitry coupled to the output of the AM-Detector 200.
FIG. 2B illustrates a prior art full wave bridging rectifier AM-Detector 210 using NMOS devices M1-M4 that essentially function as diodes. The rectifier 210 of FIG. 2B functions in substantially the same way as the rectifier 200 in FIG. 2A.
FIG. 2C illustrates a prior art cross-connected NMOS full wave bridging rectifier AM-Detector 220 using NMOS devices M1-M4. The rectifier 220 of FIG. 2C functions similar to the rectifier 210 in FIG. 2B, with the additional benefits of increasing sensitivity provided by these cross-connected NMOS devices of M1 and M3.
However, both zero-bias Schottky diodes and zero-threshold MOS devices are sensitive to temperature, and are affected by process variations. More particularly, even with these feature-added devices, the conventional RF AM-Detector circuits suffer from the receiving sensitivity loss caused by ambient temperature changes and the foundry's inherent process variations. For instance, one degree Centigrade (C) temperature decrease will normally cause either the forward bias voltage of a Schottky diode or the threshold voltage of MOS devices to increase by about 2 mV. The forward bias voltage of a Schottky diode or the threshold voltage of a MOS device could potentially vary by more than 200 mV over a 100° C. temperature range, a large issue where the device is attempting to detect millivolt signals. This problem is an ongoing concern, as many commercial products are used in temperatures ranging, for example, from −5° C. to 70° C. in outdoor and refrigerated or heated environments; while industrial and military products cover even wider temperature ranges such as −40° C. to 85° C. and −55° C. to 125° C., respectively, in applications from cold storage to heated processing environments to high altitude and outer space applications.